One of the main problems when fabricating lead frames for integrated circuits concerns the need for optimizing system performances while, at the same time, reducing or, at least restraining, production costs.
Lead frames consist of a metal substrate, also known as base metal, usually covered with one or more covering metal layers.
The substrate is usually made of copper, copper alloys, steel, iron and nickel alloys, or nickel and steel alloys (invar). The substrate is produced by a metal sheet, which is patterned by means of punching, cutting or chemical etching, so as to form the components of the lead frames such as the conducting segments and the area for housing the semiconductor devices. These semiconductor devices are then electrically coupled to the conducting elements by means of wire bonding, and are mechanically mounted onto the housing area of the system by means of encapsulation with plastic materials or resins.
Lead frames typically have thus to guarantee both high solderability for implementing electrical connections, and optimal adhesion to the encapsulating material that encloses the semiconductor devices.
It has been observed that, at least sometimes, the substrate is not able to possess these properties. For instance, in the case of a copper substrate, formation of corrosion products such as oxides or sulphides on the substrate surface has been observed. The presence of these corrosion products deteriorates the substrate solderability.
For this reason, the idea of plating the substrate with one or more covering metal layers has been put forward, so as to guarantee clear areas with excellent and stable solderability. In particular, precious metals such as palladium, silver, and gold have been used for forming coating layers with high and stable solderability. Examples of such structures for lead frames can be found in EP0335608B1, which is incorporated by reference.
However, using these precious metals for coating the substrate has caused the cost of lead frames to increase dramatically. In particular, coating the entire surface of the substrate of lead frames with one or more layers of precious metals, such as palladium or gold, requires considerable amounts of such metals.
In order to overcome this problem, a procedure for selectively depositing the coating layers (selective plating) has been suggested. In particular, due to this approach, the coating layers of precious metal are deposited only on predetermined areas of the substrate, so as to reduce the system surface occupied by precious metals, while guaranteeing high solderability only in those areas which are actually designed for bonding, for example only at the end of the conduction means. This enables up to 60%-75% saving of precious metal used with respect to the configuration with total coating. An example of a configuration wherein palladium is selectively deposited in predetermined areas of the substrate can be found in U.S. Pat. No. 7,064,008B2, which is incorporated by reference. A further example can be found in U.S. Pat. No. 7,504,712B2, which is also incorporated by reference.
Although the selective plating of precious metals has enabled a reduction in lead-frame manufacturing costs, since the amount of precious metals used has been reduced, the methods used so far for implementing the selective plating may display several disadvantages and problems.
One of the methods initially used for implementing the selective plating of precious metals is based on mechanical screening systems (for example metal masks) adapted to screen those substrate areas which do not have to be coated with precious metals and, thus, to leave those areas which have to be coated with precious metals free (i.e., exposed). However, by using these mechanical masks, a high degree of precision typically cannot be guaranteed, since the precious metals that are deposited by plating are likely to leak through the gaps which may inevitably form between the mask and the substrate. This causes both the plating to be inaccurately implemented and the precious metals to be wasted.
Moreover, using mechanical masks carries the risk of damaging the lead frame substrate owing to the pressure exerted by the mask on the substrate.
In order to overcome problems arising from using mechanical masks, the proposal has been put forward to produce masks made of a photoresist and to use laser light for exposing those substrate areas upon which the coating layers have to be selectively deposited. An example of such a solution can be found in U.S. Pat. No. 4,877,644A, which is incorporated by reference.
However, using photoresists is costly, since these materials are costly. Furthermore, the procedure for removing the photoresist from the predetermined areas in which the plating is performed is lengthy and slow and, thus, it considerably slows down the speed of lead frame manufacturing. Furthermore, in these techniques, the relative motion between the laser beam and the photoresist-covered-substrate is achieved by moving the substrate and keeping the direction of the laser beam fixed. Due to this reason, this method cannot be used for producing lead frames according to reel-to-reel or strip-to-strip processes.
In order to obviate these problems, in WO 00/52231, which is incorporated by reference, the option of using low-cost electrophoretic materials for implementing the mask has been proposed. According to WO 00/52231, the layer of electrophoretic material is selectively removed by driving a laser beam with wavelength of approximately 400 nanometers (nm) to 1200 nm across the surface by means of an optical galvo system. The method described in WO 00/52231 is nevertheless slow and decreases the speed of production of lead frames.